In web coating, printing and drying operations, it is often desirable that the web have contactless support, in order to avoid damage to the web itself or to the wet coating (such as ink) previously applied to one or more surfaces of the web. One conventional arrangement for contactlessly supporting a web during drying includes horizontal upper and lower sets of air bars between which the web travels. Hot air issuing from the air bars both dries and supports the web as it travels through the dryer.
The hot web subsequently must be cooled. Prior art devices have cooled via conduction or convection which could be either too fast or too slow, causing product quality problems, such as loss of gloss, buildup of ink on web path rollers, or generation of smoke from continued solvent evaporation. Existing methods of mitigating these problems have led to undesirable expenditure in terms of capital cost for additional or larger web cooling equipment, or reduced productivity and efficiency by having to run at slower production speeds. Other prior art devices cool the web primarily via evaporation of liquid, rather than through conduction or convection, thereby allowing moisture availability to the web, which for example in the case of a printed paper web, minimizes web shrinkage, and minimizes static electricity in the web. This can be advantageous, since the paper web, in an offset dryer, is typically dried to less than 2% moisture; and therefore, absorbs water from room air bringing its moisture level back to 4-6%. This absorbance of moisture from room air is slow, taking hours or days as the printed product is typically stacked or wound on rolls, which in this form presents limited surface area exposed to the room air. The addition of moisture may be accomplished more readily by the direct contact to a liquid water source prior to stacking or winding. Such systems are offered commercially by Weko (application by a contact roller) or Eltex (spray application).
Webs printed using the heat set web offset lithographic printing process typically require a slip agent such as silicone oil, such as polydimethylsiloxane (PDMS), to be emulsified in water and applied to the surface of the web prior to winding the printed web into rolls, or more commonly, prior to cutting, folding and stacking into books. This slip agent provides for improved handling characteristics of the printed web to resist scuffing and offsetting (mechanical transfer) of ink from the web surface to path roller surfaces, transfer belts, fold formers, nip rolls and the like, or to the facing page surfaces of a wound web or folded book. The current practice of applying silicone most often requires a prior step, which is the cooling of the web. This cooling step reduces the temperature of the web, which typically exits from the drying oven at temperatures ranging from 120 to 150° C., down to temperatures near room ambient, approximately 25 to 35° C. Application of water-based silicone emulsion is typically conducted after the web has been cooled by conductive contact with a series of cooled rollers (chill rollers). In some cases, silicone is applied while the web is still at elevated temperatures in order to take advantage of evaporative cooling, which is less costly than cooling entirely by conduction to rollers chilled with water. A known advantage of this more recent practice is that it tends to keep the chill rollers as well as the downstream path rollers free of ink deposits. Such a process is disclosed in U.S. Pat. No. 5,471,847. However, this application to a hot web has the disadvantage that the solution applied to the surface loses varying amounts of water to evaporation, depending upon incoming cooling load required owing to web temperature, line speed, and web weight. Consequently, sufficient silicone fluid must be applied to the web in order to achieve the desired amount of evaporative cooling in the most demanding conditions for web cooling, such as high incoming web (dryer exit) temperatures, high web speeds or heavy web weights. This results in consumption of excess silicone concentrate fluid to cool the web, which is costly in terms of silicone material consumed, and may in some cases adversely effect the quality of the printed ink surface causing reduced gloss, fluid streaks, or sticking of pages from excess silicone material applied.
One potential solution to this problem is disclosed in U.S. Patent Publication No. 2004/0173149. It discloses mixing the silicone concentrate and water “on the fly” in response to web conditioning requirements. However, it is difficult in typical press room operations to set up and keep such a system stable during actual production conditions as feedback control means for monitoring the amount of silicone application are not practical, and “recipe” type setups on an a priori basis require testing, adjustment and control plans for each production variation in speed, temperature, web weight and paper type.
The present invention substantially overcomes these and other shortcomings.